Monthly Variance that has gaps in monthly data

Hello,

I am having a strange issue in calculating the difference between the Avg sales of a given month vs that of the last available month.

eg:

Jan - Avg Sales - 13

Feb - Avg Sales - 12; Variance =  -1

Oct - Avg Sales - 15: Variance = 3

In this case, i want to calculate the sales variance on a monthly calendar/months as X-axis and highlight/color code the variations only.

the months is between can be left blank as there were no sales in those months.

it will be great help if you can help me with the solution.

thanks,

Ranjit